1. Field of the Invention
The present invention relates to a power storage device including an ionic liquid (also referred to as “room temperature molten salt”).
Note that the power storage device refers to all elements and devices which have a function of storing power.
2. Description of the Related Art
A lithium secondary battery which is one of the power storage devices is used in a variety of applications including mobile phones and electric vehicles (EV). Characteristics such as high energy density, excellent cycle characteristics and safety under a variety of operating environments are necessary for the lithium secondary battery.
Many of the widely used lithium secondary batteries include a nonaqueous electrolyte (also referred to as “nonaqueous electrolyte solution”) including a nonaqueous solvent and a lithium salt containing lithium ions. An organic solvent which is often used for the nonaqueous electrolyte is an organic solvent which is liquid at ordinary temperatures and pressures, such as ethylene carbonate which has high dielectric constant and excellent ion conductivity.
However, the organic solvent has volatility and a low flash point. For this reason, in the case where a nonaqueous solvent including the organic solvent is used for an electrolyte solution of a lithium secondary battery, the internal temperature of the lithium secondary battery might rise because of an internal short circuit, overcharge, or the like, and the lithium secondary battery might explode or ignite.
In view of the above, the use of an ionic liquid which has non-flammability and non-volatility as a solvent for a nonaqueous electrolyte of a lithium secondary battery has been proposed (see Parent Document 1).